Much work has been done to remove pollutants from emissions from coal fired furnaces. The focus of most of these efforts has been toward the removal of particulates, nitrogen oxides (NOx) and sulfur oxides (SOx) from flue gas. Commercially available techniques for reducing SOx emissions in furnace flue gases include fuel switching, wet flue gas desulfurization (wet FGD or wet scrubbing), and dry flue gas desulfurization (dry FGD or dry scrubbing).
A significant number of coal fired furnaces switched to lower sulfur coal to avoid the expense of installing wet or dry FGD systems for SOx control. However, an equally significant number of coal fired furnaces have installed FGD, with wet FGD being far more common than dry FGD for SOx control.
The first wet scrubbers installed in the U.S. were combined particulate collectors and SO2 adsorbers. However, the energy requirements for particulate collection proved to be excessive. Today, the most popular wet scrubber design is the spray tower.
Flue gas enters the side of the spray tower module at a temperature of 250° F. to 350° F. and is evaporatively cooled to approach its adiabatic saturation temperature by a slurry spray. Spray nozzles are used to control the mixing of the slurry with flue gas. Typically, large numbers of small nozzles are used to minimize flow maldistributions. The bottom of the spray tower contains a large tank called the reaction tank or the recirculation tank. The large volume of this tank provides sufficient residence time to allow the chemical and physical SO2 retention processes to approach completion.
Most wet FGD scrubbers in the American utility industry operate with slurries of lime and magnesium salts or limestone. The slurries are atomized into fine sprays that maximize the interfacial area between flue gas and the aqueous phase to achieve typical SO2 sorption efficiencies above 80%. Whereas these processes do not achieve the even higher sorption efficiencies predicted for thermochemical equilibrium of the sulfur-species, they do achieve the equilibrium retentions of oxidized mercury species when pH is maintained higher than 4.5. Such retentions have been found to vary from 60% to 99% of the oxidized mercury at the wet FGD inlet, depending on flue gas composition.
Dry scrubbing is the principal alternative to wet scrubbing for SO2 control on utility boilers and is often used on units burning low sulfur coal. Dry FGD scrubbers operate with slurries of lime or calcium oxide that are also atomized into fine sprays and injected into flue gas. All the water is evaporated by the thermal energy of flue gas during sufficient residence time for SO2 and other acid gases (HCl, SO3) to react with hydrated lime, which forms a mixture of solid calcium compounds (CaSO3, CaSO4, CaCl2). Since all the water is evaporated, the mixture of the ultimate reaction products is dry and amenable to particulate removal by electrostatic precipitators (ESPs), baghouse filters (BHFs), cyclones (CYCs), and other similar devices. Most of the oxidized and particulate mercury compounds in flue gas at the inlet to the dry FGD are also retained on the dry product mixture. Test data show that the retention of mercury in dry scrubbers increases in direct proportion to the sum of oxidized and particulate-mercury at the inlet, and therefore varies widely among different coal fired furnaces.
While the art has focused primarily on the removal of NOx and SOx from flue gases, there are also concerns about emissions of mercury and other elemental metals such as chromium, arsenic and lead from combustion devices. Mercury (Hg), the eightieth element, is an important pollutant which must be controlled. As a vapor it is a poison to the nervous system. It is the dire consequences of chronic mercury poisoning which gave birth to the term “Mad as a hatter.” Hatters that used mercury to block the hats were exposed to toxic levels of mercury vapor. The tremors, shakes, stutters, and stammers common to mercury poisoning were endemic in the trade. Neither were astronomers, who frequently used telescopes which were floated on mercury, immune from this disease. It was at times fatal and has the characteristic of being cumulative over years of exposure, as the body's nervous system has difficulty in purging this element. Most industrial uses of mercury today are carefully controlled. The biggest anthropogenic sources of environmental mercury are coal combustion and the combustion of municipal solid waste. Coal, and especially municipal solid waste compositions, may also result in emissions containing chromium, arsenic and lead.
Mercury vapor is a poison. At the levels common in the atmosphere the concentrations are usually safe. However, mercury accumulates in lakes where it is further accumulated in fish. These fish, with organic mercury molecules in them, can be hazardous to individuals who eat them. Some states request and have posted warnings that people eat fish from some lakes no more frequently that once a week. Often it is stated that pregnant women and small children should eat no such fish.
Several states and the United States Environmental Protection Agency will soon limit the emissions of mercury and possibly other trace metals from combustion devices. A method for controlling emissions of mercury and other metals is therefore needed. Some control of mercury levels is possible by using particulate collection devices. However, only very expensive baghouses (fabric filters) are efficient enough to reduce the mercury to levels that may be required, and still it is possible for the elemental mercury vapor to escape as a gaseous vapor molecule.
Activated carbon and other fine particulates are used to absorb mercury. Special treatment of the activated carbon has been tested. However, collection by the use of activated carbon is very expensive. So, it is seen that a new method of removing mercury from flue gas is needed.
The present invention is directed toward overcoming one or more of the above-mentioned problems.